This study demonstrated at the cellular level that high glucose concentration can promote the expression of ICAM-1 and MMP-9 proteins in HUVECs in a concentration-dependent manner. ART can inhibit the expression of ICAM-1 and MMP-9 proteins under high glucose conditions, and the inhibitory effects are also concentration-dependent.
There is increasing evidence that inflammation plays an important role in the pathogenesis of DR. DR begins as a low-grade chronic inflammatory disease [15]. During the development of DR, pro-inflammatory factors, chemokines, and leukocyte adhesion increase [16], and leukocyte stasis is a major component of the inflammatory process [17]. ICAM-1 expressed on endothelial cells can not only regulate leukocyte adhesion to endothelial cells but also play an important role in regulating blood-retinal barrier disruption and vascular permeability [18-20], which can induce retinal capillary non-perfusion and neovascularization [13,14]. MMP-9 can promote leukocyte stasis [21], and its mechanism is related to basement membrane degradation and leukocyte aggregation at the site of tissue damage [22].
In addition, the stages of neovascularization in DR include basement membrane degradation, endothelial cell migration and proliferation, followed by capillary formation. The migration and remodeling of this tissue are regulated by matrix metalloproteinases (MMPs) [17]. MMP-9 is the largest member of the MMP family [23], which can promote increased vascular permeability [24], degrade the capillary basement membrane, a part of extracellular matrix [7], and is a necessary condition for endothelial cells to infiltrate and form new lumens in the subendothelial matrix [8]. Moreover, MMP-9 can degrade the tight junction protein components of the BRB endothelial cells, increasing vascular leakage [25].
Previous studies have shown that the expression of ICAM-1 and MMP-9 is increased under high glucose conditions in endothelial cells and DR patients [8] [26-28], and the increase in ICAM-1 expression on endothelial cells is glucose concentration-dependent [29]. The results of this study are consistent with previous research. In addition, this study further demonstrated that with the increase of glucose concentration, the expression of MMP-9 in endothelial cells will also increase, indicating that higher blood glucose levels may worsen the severity of diabetic retinopathy. The results showed that ART can inhibit the expression of ICAM-1 and MMP-9 proteins in endothelial cells under high glucose conditions, and the inhibitory effects were concentration-dependent. ART can suppress iris and retinal neovascularization in rabbits and alleviate macular edema in monkeys by downregulating the expression of VEGFR2, PKCα, and PDGFR [6].
This study provides new targets and theoretical support for ART in inhibiting the formation and leakage of retinal neovascularization. In addition to high levels of ICAM-1, the levels of its ligands CD11a/CD18 and CD11b/CD18 are also increased in diabetic patients [30]. Blocking the expression of ICAM-1 or CD18 reduces leukocyte stasis, endothelial cell death, and vascular leakage in the retina of diabetic animals [31]. In DR mouse models with ICAM-1 and CD18 genes knocked out, the number of leukocytes adhering to retinal vessels decreases, the number of endothelial cell injuries decreases, the breakdown of the blood-retinal barrier is reduced, and the histopathological changes in retinal vessels are alleviated [16]. Inhibiting MMP-9 activity can suppress corneal neovascularization and protect the integrity of BRB function, reducing retinal vascular leakage [32,33]. Therefore, based on the results of this study, ICAM-1 and MMP-9 may become new targets for treatment of DR by ART and play a therapeutic role.
In the pathogenesis of DR, vascular endothelial growth factor (VEGF) is the main factor that promotes the formation of neovascularization and macular edema [34]. However, other factors such as ICAM-1, MMP-9, HMGB-1 [35-37], TNFα [38, 39] also play similar roles. Therefore, VEGF is not the only factor for neovascularization and leakage. Currently, the treatment for neovascularization and macular edema mainly involves the use of anti-VEGF drugs. Anti-VEGF drugs can inhibit neovascularization and leakage and improve visual acuity in patients with macular edema [34,40]. However, anti-VEGF drugs have certain limitations. For example, they have a short half-life [41], and the duration of drug action is short. For instance, widely used drugs such as ranibizumab require monthly intravitreal injections to maintain therapeutic levels of the drug in eye tissues [42]. They can only target some subtypes of VEGF. Aflibercept and conbercept are able to target placental growth factor [41]. The most important limitation is that less than 50% of patients receiving anti-VEGF treatment achieve an improvement in vision [43-46]. Studies suggest that strict control of blood glucose can reduce the incidence of blindness caused by DR [47], but it is difficult to maintain normal blood glucose levels in clinical practice, and sometimes even impossible. Therefore, more treatment targets need to be elucidated based on the molecular mechanisms of this disease to prevent and treat it effectively. As ART can inhibit the occurrence, development, and leakage of retinal neovascularization by acting on three targets, VEGFR2, PKCα, and PDGFR, the drug has a long duration of action, lasting up to 6 months [6]. In addition, this study demonstrated that ICAM-1 and MMP-9 can serve as two new targets for ART at the cellular level. Furthermore, ART has good tolerance [48]. The concentration of artemisinin and its derivatives required for anti-neovascularization effects is only one-thousandth of the clinical dose used for malaria treatment [49]. Under long-term systemic administration conditions of low doses (10mg/L), there are no additional side effects [47,50-54]. Therefore, ART may be used for early and late-stage treatments of DR.
In conclusion, this study demonstrated that the increase in ICAM-1 and MMP-9 is glucose concentration-dependent in cell models, and based on previous research, it suggests that DR patients should strictly control their blood glucose level to some extent. Furthermore, ICAM-1 and MMP-9 may serve as new targets for ART in the treatment of DR, providing information for further research on its potential therapeutic value as a new probable type of drug for DR. However, this experiment is limited to the effect of ART on the expression of ICAM-1 and MMP-9 at the cellular level. Further research is needed to investigate the changes in ICAM-1 and MMP-9 expression involved in the mechanism of inhibition of ART of retinal neovascularization and leakage.